The present invention relates to a method of production of a coating composition and also to the coating itself, which is laid on top of an optical fibre for covering it or safeguarding it. It is of special, but not exclusive application, for manufacturing the covering for optical fibre.
A process for producing a composition useful for coating a surface such as a glass fibre, an optical fibre or similar, is known through the patent U.S. Pat. No. 3,862,074 xe2x80x9cAqueous polymeric compositions of non-ionic urethane polymer and acrylic polymerxe2x80x9d, granted to Richard M. Hichey, and incorporated in the present patent application by reference.
The patent mentioned describes a polymeric composition constituted by a suspension of a polymer in a fluid such as water, that is, the composition employed for coating, for example, an optical fibre is a liquid which has in suspension extremely small particles of the aforementioned polymer, insoluble in the water.
Once the polymeric composition useful for coating surfaces is deposited on the surface mentioned it is subjected to a curing process during a period of 15 to 30 minutes.
The patent U.S. Pat. No. 5,925,462 xe2x80x9cAqueous coating compositions for glass fibers, fiber strands coated with such compositions and optical fiber cable assemblies including such fiber strandsxe2x80x9d, granted to Mikhail M. Girgis, and incorporated in the present patent application by reference, describes a process for production of a double cover or coating for optical fibre.
The composition useful for coating the optical fibre is formed by a double coating, such that the first coating is obtained from an aqueous emulsion or dispersion of an acrylic polymer in a first aqueous solution, and the second coating is obtained from an aqueous emulsion or dispersion of another acrylic polymer, different from the previous acrylic polymer, in a second aqueous solution. The second acrylic polymer is free from urethane.
In the aforementioned, the double coating is formed by an internal layer and an outer layer, both of them made from UV (ultra violet) cured resins.
In such structure, the internal layer having a glass transition temperature (Tg) of no more than about xe2x88x9220xc2x0 C. and preferably about xe2x88x9230xc2x0 C. or even lower, and a Young""s modulus lower than 2 MPa, preferably 1 MPa or even lower, prevents microbending and an external layer with a relatively high Tg, generally approximately 50xc2x0 C. and a modulus of about one hundred times higher provides good resistance for fibres handling and cables manufacturing process and installation.
In the prior art, these two layers are not obtained from emulsions but from 100%-solid UV curable resins; they can contain acrylate, urethane, ester, etc.
When UV curable resins are used, the liquid resins are applied on the surface fibre and then a curing process is applied to both layers of coating deposited on the surface of the optical fibre.
The curing process is carried out at a relatively low temperature and lasts for several minutes. As a consequence, there is a need to reduce the production costs of optical fibres, for example through a modification in the curing process of the composition used to coat the optical fibre.
The curing process of the invention guarantees that the covering, at least, retains the characteristics to be found in optical fibre coatings obtained through the slow curing process.